This invention relates to a new process for the production of weather-resistant molded foams having high dimensional stability under heat.
Foams based on polyisocyanates, for example polyurethane foams having an impervious outer skin and a cellular core, of the type obtained by the in-mold foaming technique described in German Auslegeschrifts Nos. 1,196,864 and 1,694,138 and French Pat. No. 1,559,325 are particularly suitable for the mass production of lightweight constructions. These foams may be used in furniture, motor vehicles, home construction and elastic moldings, such as cushions, shock absorbers and shoe soles.
The polyurethane moldings are produced by introducing the foamable reaction mixture, which generally contains polyisocyanates, compounds containing at least two isocyanate-reactive hydrogen atoms and additives, into closed, temperature molds in which it foams and hardens. The plastics material completely fills the mold and accurately reproduces the inner surfaces thereof.
The starting materials previously available on the market and the processes previously applied in practice for the production of rigid polyurethane foams are based on the crosslinking of aromatic polyisocyanates with polyether or polyester polyols. Moldings such as these have a high level of mechanical properties, such as E-modulus, thermal stability under load and impact strength, which makes them suitable for numerous applications, such as furniture components, industrial housings and window profiles. One disadvantage common to all of these moldings is that they turn yellow in color as a result of weathering and undergo surface roughening through chemical degradation. As a result, the moldings generally must be coated with a protective lacquer in order to meet more stringent requirements.
It is known that polyurethanes, and particularly polyurethane lacquers, based on aliphatic or alicyclic isocyanates do not have the above-noted tendency towards yellowing. The use of aliphatic or cycloaliphatic polyisocyanates for the production of molded polyurethane foams, and particularly rigid polyurethane foams, with an integral density distribution has generally been avoided since the resultant rigid molded foam is characterized by inadequate mechanical properties. In particular, the dimensional stability of rigid molded foams under heat tested in accordance with DIN 53 432 (based on the classical aliphatic polyisocyanates, such as for example hexamethylene diisocyanate or the biuret polyisocyanate corresponding to this diisocyanate) did not satisfy the requirements of practical application in this respect.
Accordingly, an object of the present invention is to provide a new process by which it is possible to produce rigid molded foams based on polyurethanes which are not attended by the disadvantage of inadequate weather resistance and which show in particular high dimensional stability under heat in addition to a high E-modulus and high impact strength.
According to the invention, this object may be achieved by using certain, selected polyisocyanates in the production of the polyurethane foams.